It is known to polymerize ethylene with other .alpha.-olefins and optionally with unconjugated dienes or to polymerize olefins or conjugated dienes alone in the presence of organometallic (Ziegler-Natta) mixed catalysts (Encycl. Polym. Sci. Eng., 2.sup.nd edition, volume 6, pages 522 et seq., Wiley, N.Y. , 1986). Polymerization is performed in solution, as a suspension or in the gas phase. The catalysts used are transition metal compounds of subgroups IV to VI of the periodic system (usually vanadium compounds in valence state +3 to +5) together with organometallic compounds of main groups I to III (usually organoaluminum compounds). Such catalytic systems exhibit very high initial activity, which, however, quickly falls due to the rapid reduction of the transition metal to low valence states (for example +2), which are inactive for polymerization purposes. Reactivators, which reoxidize the transition metal compound to return it's valence states which are active for polymerization purposes, are accordingly used in order to increase polymer yields (for example expressed as the quantity of polymer formed in g per g of transition metal).
The most efficient reactivators for catalysts containing vanadium are substances containing chlorine. Polychlorinated compounds, such as for example trichloroacetic acid esters (DE 1,570,726), perchlorocrotonic acid esters (DE 1,595,442) or hexachlorocyclopentadiene (DE 1,495,698) have proven to be effective in practice. However, these reactivators have the disadvantage that the resultant copolymers exhibit a very high chlorine content. The chlorine content has a negative effect on many polymer properties, primarily aging resistance. Moreover, polymers containing chlorine give rise to increased corrosion on plant components during working up of the copolymer after polymerization and on processing plant. Compounds having a lower chlorine content, for example mono- and dichloromalonic acid esters (CA 272,857, DE 2,344,267), usually exhibit low activity. In practice, this results in a low solids content in the polymer solution. Remedying this deficiency would require disproportionately large excesses of reactivator relative to the vanadium compound, which is economically disadvantageous. Effective compounds having a lower chlorine content have been described as reactivators in recent years, for example dichlorophenylacetic acid esters (EP 0,044,119 and 0,044,595). However, even when such reactivators are used, the chlorine content in the polymer may only be reduced to the necessary low level by means of costly polymer washing.
EP 0,680,976 discloses the use of arylhalomalonic esters as reactivators for Ziegler-Natta catalysts containing vanadium. While these compounds are indeed effective, in comparison with dichlorophenylacetic acid ethyl ester, twice the quantity of reactivators must be used in order to achieve satisfactory yields.